Network design tools are offline tools used to plan and deploy networks, such as optical networks. A central goal of any network design tool is to come up with an optimal network that can carry the given set of traffic demands. The design process involves routing the demands subject to one of several optimization metrics, such as the dollar cost of the network or the path lengths of the demands. The resulting design is used in multiple ways. Equipment vendors use the design in responding to requests for proposal (RFP). Carriers use the design to deploy their network and conduct what-if analyses.
In the latter case, once the network has been deployed according to the design, an online system known as a network management system (NMS) is responsible for routing the demands as they arrive in time. Typically, point-and-click software is used to automate this process over mesh networks. The NMS software is typically responsible for computing the optimal routes for the demands and signaling the cross connects to correctly set up the lightpaths. One example of NMS software is a product called WaveStar™ SNMS available from Lucent Technologies, Inc. (Murray Hill, N.J.).
Clearly, it is expected that the design network is operationally effective, i.e., is able to successfully carry the traffic for which it was designed. While this may seem like a foregone conclusion, there are certain disparities or mismatches between the design phase (offline route planning) and operation phase (online NMS-based routing) that make this difficult to achieve in practice.
First, the optimization criteria used to route demands during the two phases are often unrelated. For example, the design algorithm may be routing to minimize costs, while the NMS may be routing to minimize network congestion. As a result, the design algorithm may allocate capacity for a demand on certain links, whereas the NMS is trying to route the demand on different links. In some cases, this may result in not being able to route a demand.
Second, the order in which the demands arrive may be different from the order assumed during design algorithms. Since routing typically depends on the current usage of the network (e.g., in congestion-based routing), the computed paths may once again be different in design and operations, adding to the above problem. Clearly, failure to route a demand when there is stranded capacity in the network can be catastrophic to the bottom line of a network carrier. The solution adopted in practice is to force the NMS to route the demands along the paths computed by the design algorithm. However, this is not satisfactory since it ignores any operational criteria in routing.